Not sure what TRAP does in BASIC, but the TRAP
instruction in the assembler manual that you linked generates a hardware exception that can be handled by the operating system.
There is rarely need for a programmer to use this instruction in their code. The typical use of it is to be inserted by a debugger into the code being debugged at the point where a stop is desired (breakpoint), then running/continuing the program, and regaining control once the TRAP
instruction is reached.