If you ask most people what a hypothesis is, you'll probably get something along the lines of: "an educated guess." While that is technically true, that answer only covers part of what a hypothesis actually does in real research.
It's also worth clearing up a mix-up that trips up a lot of students down the line: a hypothesis is not the same thing as a theory. In everyday language, "theory" means an unproven guess ("I have a theory about why the WiFi keeps cutting out"). In science, it means the opposite: a theory is a well-substantiated explanation, supported by extensive evidence, that has survived repeated testing.
Scientists also typically frame two competing hypotheses before running a study: a null hypothesis (there's no real effect or difference) and an alternative hypothesis (there is one). The entire experiment gets designed specifically to test which one the data ends up supporting.
A hypothesis is more than a guess. It's a specific, testable prediction about the relationship between two things. "Sunlight affects plant growth" sounds like a hypothesis, but it's actually too vague to be useful. Questions like what counts as "affects"? Faster growth? Taller plants? Greener leaves? Confirm that this is not a hypothesis. Moreover, how would you ever know if you were wrong? A real hypothesis looks more like: "Plants exposed to 8 hours of sunlight per day will grow taller over two weeks than plants exposed to 4 hours." That version is specific, measurable, and falsifiable. Falsifiable is the key word here: the point of writing down a hypothesis is to determine if you were right or wrong about something — in short.
Most hypotheses follow an "if/then" logic, even when they're not written using those words explicitly. For example: if [something changes], then [something measurable will happen], because [your reasoning why]. The reasoning section is often underlooked by students, but a hypothesis backed by a clear rationale (existing research, a known biological mechanism, prior data) is much stronger than one that's just a hunch.
There's also a distinction between directional and non-directional hypotheses. A directional hypothesis predicts which way the effect will go ("plants will grow taller with more light"), while a non-directional hypothesis just predicts that there will be some difference, without committing to a direction ("the amount of light will affect plant height, in some way"). Directional hypotheses are more common in student projects, mainly because they usually come from a specific, reasoned prediction rather than an open-ended question.
If you're applying to a research program that asks you to propose a project, they’ll likely pay careful attention to your hypothesis, checking whether it's specific enough to actually test with the time and resources you'l1 realistically have. A testable hypothesis is often the single biggest difference between a research proposal that gets accepted and one that gets sent back for revisions.