I heard that one should not shower after going to the mikva. What does this mean?

The prevalent custom is that a women doesn’t shower immediately after going to the mikva. This is based on the gemara (Shabbat 14a) that people were getting confused and thought that it was really the shower which is metaher (purifies) the person and not the mikva, and therefore chazal decreed that one shouldn’t wash themself after going to the mikva.

This only applies to showering while in the mikva building. Once she has arrived home, she may shower as usual.

Also, in a time of need (for health reasons and the like), she can be lenient in this and may shower even in the mikva building itself.

See Rama and poskim Y.D. 201:75