What do philosophers mean when they say that beliefs need justification?
Under the various Theories of Justification, a belief is not held as philosophically valid unless it can be justified. This means that the belief should have a reason to be held beyond simple faith or guessing; the reason should be as objective as possible and should be immune to most criticism or challenge. This is, of course, very difficult; most people believe things without thinking about the underlying reasons behind them. Adding to the difficulty is that there is more than one Theory of Justification; each has a slightly different take on how to "properly" justify beliefs, and which justifications are valid. For example, under Coherentism, beliefs are valid if they are in line with other beliefs held by a person (as in many religious beliefs that hold to a strict interpretation of texts and moral philosophy), whereas with Internalism, a person should be able to justify their beliefs through personally-held knowledge, not relying on outside facts or the opinions of others. This makes it hard to pin down a specific, overall Theory of Justification, but in general if a belief can be justified with specific reasons, it can be held as valid. A belief that is justified only with an explanation, or with illogical reasoning, cannot be held as valid.