A circular reference occurs when a formula depends on itself, either directly or through other cells. A total that includes its own cell is a common cause. Excel warns about the circle because the result can't be calculated in the usual dependency order.
First inspect the formula range. A total in B5 should usually sum B2:B4, not B2:B5. An indirect circle needs more tracing: if B4 reads B5 and B5 reads B4, neither cell has an independent starting result.
Some financial models deliberately use feedback. Enable iterative calculation lets Excel repeat calculations until it reaches Maximum Iterations or the change between results falls below Maximum Change. Iteration is a modeling choice, not a general repair for a mistaken range. Fix accidental self-references before changing calculation settings.