What does Rollback mean in a concurrent environment?

What does Rollback mean in a concurrent environment?

This question is about T-SQL/SQL Server.
Suppose we have a transaction that inserts some row X1 into table X,
updates row X2 in table X, and then issues a rollback.
Right before the rollback, a second transaction updates rows X1 and X2 in
table X and commits. (I suppose that in order to update X1, that
transaction must have isolation level Read Uncommitted.)
What is the state of rows X1 and X2 after the first transaction's rollback?