Remainder theorem f(x)= 2x^3 - 3x^2 - 5x + 23 is divided by x+2 explain in detail. ive been trying to understand this and i keep confusing myself even more.

The remainder theorem lets you find the remainder when a polynomial is divided by a linear function like the above, without actually doing the division.

It states that  when the polynomial f(x) is divided by x - a  then the remainder is f(a)

Applying it to the items in your question  x - a = x + 2 so -a = 2 and so a = -2 :-
f(-2) = 2(-2)^3 - 3(-2)^2 - 5(-2) + 23 =  5  so the remainder will be 5.

Note if the remainder had been 0  then  x + 2 would have been a factor of f(x). This process can be used to find if x - a is a factor.