MTSS" stands for Multi-Tiered System of Supports, and it's a plan to help all students do well in school. It has different levels or tiers to help students with different needs.
Tier 1 is like the base. It's the regular stuff all students get from their regular teacher. Teachers change their teaching a bit to help students who need it, making things easier or harder.
Tier 2 is for students who need extra help. They get special lessons in small groups to practice and get better at things.
Tier 3 is for students who need even more help. They get really special help that's just for them.
The goal is to help every student do their best in school. Teachers use data to figure out which students need which level of help. It's kind of like building a strong foundation for a big building.