This page will show you how to read the front of the green sheet which is provided in the front of the textbook. If all else fails, consider what the same thing would mean in C.
foo = bar
assigns foo to bar, just as in C syntax.
R[rs]: Refers to the value in the register specified by the rs field of the instruction
M[foo]: Refers to the memory location specified by the address that foo evaluates to.
M[1549] refers to the element stored at address 1549 (and I'd better be doing byte-wise addressing if I use that address).
baz = foo + bar: Means you add foo and bar and assign baz to be the result.
This is the sign extended immediate value (see the bottom left-side of the green sheet).