Conclusion
A conclusion is the statement of what an investigation showed, based on the evidence, including whether the hypothesis was supported.
Definition
A conclusion is the answer to your science question based on your data: it says whether the hypothesis was supported and what the results showed. A full conclusion restates the hypothesis, cites the specific evidence (means, differences, patterns), explains the result with scientific reasoning, and notes limitations, sources of error, and what could be tested next. At its most rigorous it states the size of the effect, compares that effect to the measurement variability, distinguishes what the data show from what they merely suggest, identifies systematic errors and confounds that could alter the interpretation, and specifies the conditions under which the result should hold. Overreaching beyond the design is the most common flaw.
Example
"My hypothesis was supported: the window plants grew an average of $4\ \text{cm}$ more than the closet plants, so sunlight helped the bean plants grow." "The data supported the hypothesis that warmer water dissolves salt faster; mean time fell from $48\ \text{s}$ at $10^\circ\text{C}$ to $14\ \text{s}$ at $50^\circ\text{C}$, and a limitation is that the $50^\circ\text{C}$ water cooled during the trials, explaining the larger range there." "Across five angles, mean cart speed rose from $1.7$ to $3.4\ \text{m/s}$, consistent in shape with $v \propto \sqrt{\sin\theta}$ but about $8\%$ below the frictionless prediction at every angle, indicating a systematic energy loss; the result applies to this cart on this surface."
Key Insight
A conclusion has to use your data: "plants need sunlight" is a fact you already knew, while "my plants grew $4\ \text{cm}$ more in sunlight" is a conclusion from evidence. A hypothesis that was not supported still gives a valid conclusion, and explaining why is often the most interesting part. The best conclusions say what was not shown as clearly as what was, because readers trust a claim more when its author has already marked its edges.