One hectare is exactly 10,000 square metres. It is the area of a square 100 metres on a side, and the round number is deliberate — the hectare was constructed to be a convenient land unit within the metric system rather than inherited from anything older.
About the units
The hectare is the are with the prefix hecto: an are is 100 m², a square ten metres on a side, and a hundred of them make a hectare. Neither unit is formally part of SI, but both are accepted for use with it and the hectare is universal in land administration, agriculture and forestry. The square metre is the SI derived unit for area and needs no separate definition beyond the metre. The relationship between them is the clearest illustration of why metric areas are easy: a factor of a hundred in length between the metre and the hectare's side becomes a factor of ten thousand in area, and both are powers of ten.
The exact factor
The factor is exactly 10,000. A hectare is 100 m × 100 m, and 100² = 10,000. Two related conversions complete the picture: 100 hectares make one square kilometre, since a kilometre is ten times a hundred metres and ten squared is a hundred; and one are is 100 m². Anyone who has internalised that areas square their linear factors will find the whole family trivial, and anyone who has not will get every one of them wrong by the same mechanism.
Where you meet this conversion
Land registration, agricultural statistics and environmental reporting. Field sizes, forest stands and nature reserves are recorded in hectares while building footprints and plot dimensions within them are in square metres, so a single planning document routinely uses both. Crop yields are reported per hectare and irrigation or fertiliser rates per square metre, which means agricultural calculations cross this boundary constantly. Urban planning uses hectares for districts and square metres for buildings.