Imagine you are in a big band, like the ones you see in parades or at football games. Now, the band has a lot of different instruments like drums, trumpets, saxophones, and others. But not everyone knows how to play every single instrument, that's why the band has sections.
A section is made up of people who play the same type of instrument. For example, all the trumpets make up the trumpet section, all the saxophones make up the saxophone section, and so on. Each section practices their own parts separately, so they can get really good at playing them.
Now, sometimes the whole band will practice together, but sometimes just one section will practice by themselves. This is called a band sectional. The trumpet section might go off into a separate room to practice their parts without being interrupted by the other sections.
It's like when you're in school and sometimes you do activities with the whole class, and sometimes you do activities with just your small group. Band sectional is just like that, but for musicians in a band.