In TON, smart contracts are executed inside TVM (TON Virtual Machine). TVM expects instructions in a specific binary format, available here:

https://ton.org/docs/learn/tvm-instructions/instructions

These binary instructions are like assembly code, and writing them by hand is very hard. At the beginning Fift was created as a scripting language to make it easier to write smart contract codes. It has a special syntax and working with it is easier than TVM instructions, but it's still very low level and hard to use, but it's doable.

After a while, FunC was developed which is like the C language. A lot easier than assembly and very powerful, yet still considered a low level language.

There are higher level languages being developed like Tact (https://tact-lang.org/) or Rift (https://rift.skyring.io/), which will ultimately produce those TVM instructions.

You can interact with smart contracts by sending them messages encoded in the binary format expected by those smart contracts.

recv_internal is the main entry function in FunC, which expects to receive internal messages generated inside the blockchain. It expects four arguments, as TVM puts those 4 args on the stack. In FunC, you may omit any of the previous args if you are not using them.