Thick bed mortar is like the glue that holds big bricks together when you're building a wall or a floor. Imagine building with Legos. You know how you have to put the small pieces together with a little bit of glue so they don't fall apart? That's what thick bed mortar does in real life.
But, thick bed mortar is different from regular glue because it's much thicker and stronger. It's made using special materials like sand, cement, and water, and it's mixed together until it becomes a sticky paste.
When you're building something big like a floor or a wall, it's important that the bricks or tiles are really secure and don't move around. That's where thick bed mortar comes in. You spread it over an area where you're going to lay the bricks or tiles, and then press them down into the mortar. The thick bed mortar holds them in place and keeps them from wiggling around.
If you're building something really heavy like a hot tub or a swimming pool, you need even stronger mortar called "medium bed" or "high bond" mortar. These types of mortar are even thicker and stickier than regular thick bed mortar.