Byleth's Jab sequence consists of 3 hits, with both a Rapid and Gentleman finisher. The Gentleman's Jab does less damage, but sends at a lower angle which is better for setting up techchases and edgeguards. Jab is good as a combo ender from a dragdown neutral or up air, or as a fast option to keep people off of you.
Jab can set up for jab locks, although it is prone to being low profiled. The best guaranteed follow-up off a Jab lock for byleth is Down Smash, which is tight, but can be done after a Jab 1. If Usmash will kill, using that instead is recommended as it's easier to time.