Slippage happens when something takes longer than expected. For example, if your mom tells you she'll be home from work at 5 pm and it's already 6 pm, that's slippage. In projects, people make plans to finish something by a certain time, but sometimes things don't go as planned and it takes longer to finish. That's called project slippage.
Imagine building a Lego castle. You plan to finish it in two hours, but then your little brother comes and needs your help with something. You have to stop building for a while and it takes longer than expected to finish the castle. That's project slippage.
In real projects, slippage can happen for many reasons. Maybe someone got sick or couldn't work on the project for a few days. Maybe some materials or equipment wasn't delivered on time. Or maybe the project turned out to be more complicated than expected. Whatever the reason, project slippage means that things are taking longer than planned, and that can cause problems. That's why it's important to try to avoid slippage by planning carefully and being ready to adjust the plan if things don't go as expected.