Assembler in C Programming is defined as a program that converts Assembly language into machine code. It just converts assembly language or low-level codes into machine codes that could be better recognized by a specific type of processor. The assembler works similarly to a compiler. But compiler converts high-level language to machine-understandable codes.
If you wish to learn C Programming, I recommend you check out the C Programming course, from Intellipaat. Also, check out the YouTube video on Learn C Programming in 1 DAY.