To put it briefly:

Shafali Verma and Smriti Mandhana put up 292 runs for the first wicket.
Mandhana and Verma both achieved century marks.
Shafali made a double century out of her maiden century.

In-depth Synopsis:

Under Harmanpreet Kaur’s guidance, Smriti Mandhana and Mithali Raj dominated South Africa on the opening day of the single Test at the MA Chidambaram Stadium in Chennai. India elected to bat first, and the two gave the Proteas bowlers a real work-out on a superb batting surface. After a cautious start, the ladies in blue quickened their tempo and finished off the new ball after ten overs.

Shafali made her century in the second session of the day with 113 balls. After 122 balls, Mandhana achieved her century on the very next delivery. Mandhana maintained her outstanding form from the ODI series, hitting 149 runs off 197 balls (with 27 fours and 1 six) before being dismissed by right-arm spinner Delmi Tucker. Alternatively, Shafali scored a double century off 194 balls following her century off 113 balls. In the 75th over, she was eventually out when she attempted to take a single. India was leading the way at 411 for 3 in 74.1 overs by that point.

Among the records that Smriti Mandhana and Shafali Verma set were:

In Test cricket, Shafali Verma became the second Indian woman to reach two centuries. This was the first time Mithali Raj accomplished it in Taunton, against England, in 2002.
With 205 runs, Shafali Verma became the second-highest Indian woman to score in Test cricket history. The highest score is still Mithali Raj's 214 against England in Taunton in 2002.

Shafali Verma broke Janet Brittin’s 1984 record of 137 balls to complete her century in 113 balls, setting a new record for the fastest century in women’s Test cricket.

In women’s Test cricket, Shafali Verma beat Australian Annabel Sutherland, who needed 248 balls to get her double century earlier this year, to record the fastest double century.

Shafali Verma beat Sandhya Agarwal, who was 21 years and 308 days old, to become the youngest Indian to hit 500 Test runs at the age of 20 years and 152 days.

At 20 years and 152 days, Shafali Verma became the second-youngest Indian woman to hit a century in a Test match, after Mithali Raj, who achieved the feat at 19 years and 254 days.

Shafali Verma is just the third opener from India to score more than 150 runs in women’s Test cricket history. The other two were Sandhya Agarwal (1986) with 190 runs against England and Thirush Kamini (2014) with 192 runs against South Africa.

With eight sixes in a women’s Test innings, Shafali Verma set a record. Previous record holders with two sixes apiece were a few players.

With 292 runs, Smriti Mandhana and Shafali Verma broke the previous mark set in 2004 by Pakistan’s Kiran Baluch and Sajida Shah against the West Indies, becoming the greatest opening partnership in women’s Test history.

In the history of the women’s Test, their partnership is also the second-highest. Australia’s Denise Audrey Annetts and Lindsay Anne Reeler set a record against England in 1987, scoring 309 runs.
With 275 runs against South Africa in Mysore, Mandhana and Verma broke the record for the highest partnership for India in women’s Test cricket, which was previously held by Thirush Kamini and Poonam Raut.
After the illustrious Mithali Raj, Smriti Mandhana became the second Indian woman to amass more than 500 runs in all three forms of international cricket.
After Sandhya Agarwal (4) and Hemlata Kala (2), Smriti Mandhana became the third Indian woman to reach numerous century in Test cricket.

Updates

Shafali and Smriti had an incredible first half of the day, setting a record for the highest opening partnership in women’s Test cricket history with 292 runs. They accomplished this despite a number of obstacles, including misdirected tosses, sporadic misunderstandings when dashing between the wickets, small distractions, and Chennai’s humidity. Shafali scored 205 runs, capping an incredible innings.