It is, as advised, a baseball term. In general usage, it means that someone is ready to take on a challenge, someone who is willing to have a go at something which is perceived as difficult. In close-run baseball games, the final hitters have the most pressure to perform well when they step up to the plate.
Stepping up to the plate (is a baseball term as above). My impression (not only of taking a bat) is that if someone is said to be 'stepping up to the plate' then they are about to face something that they might consider fearful or pressurised.