Rate of profit has a tendency to fall over time. This tendency results from increasing amount of constant capital, i.e. all inputs other than labour, in production. The tendency of constant capital to increase results from the search for relative surplus value. There's a countertendency of course; rate of profit can increase or constant capital decrease for various reasons (e.g. one industry might see the value of its inputs decrease by annexing land to the market, another may manage to gut environmental regulations to save money on waste disposal). But there is a limit to this ability to grow, so after capital finds something to gorge on the rate of profit resumes its steady fall (setting aside financial/debt nonsense that postpones this fall until it becomes a crash).

Overproduction is when the producing capitalist produces more goods than there are buyers for. Because capitalist production runs on debt, this rapidly causes cascades of bad debts leading to the abovementioned bubble and crash. Overproduction is of course relative to demand to consume the products. Therefore it can result from 'simply' producing too much, but another factor is people not being paid enough to buy things. Overproduction can (has been) avoided by enabling massive amounts of debt (another bubble), or by creating massive consumer nations whose main purpose is to consume the good produced for surplus value in the underdeveloped nations (right now we are of course doing both, a wonderful bubble)

Both are relevant concepts, fetishising and focusing on one of them is faulty analysis. One, both or neither may be relevant in any given situation