Concrete is one of the most used construction materials in the world today. It can be placed in forms to create almost any shape or placed in flat slabs thousands of feet long and inches or feet thick, it can be structural columns and beams supporting tall buildings or bridges, or it can be the hull of a boat that floats underneath the bridge and out to sea.
Concrete is a mixture of cement, usually Portland cement, coarse and fine aggregates (think of coarse as gravel and fine as sand, though other materials may be used). When cement is mixed with water and the appropriate aggregates, it becomes plastic (fluid), and it is placed in forms (molds) to create the shape the user is building. Hydration, a reaction between the dry components of the cement and water causes the concrete to set up, or harden into a stone-like material that is the foundation (literally) of the construction industry today.
Other answers concern themselves with the change in the water to cement ratio if rain occurs before the concrete has set. Having placed and finished concrete for many years, I haven’t seen that happen unless the rain comes while the concrete is being placed. As soon as the concrete has been screeded and bull-floated, the paste is pretty much established, in fact, the process of bull floating is partly to raise paste to the concrete surface. While hard, driving rain or long sustained rainfall may disturb the paste, it doesn’t penetrate through the top layer of the aggregate unless the surface isn't sloped enough for the water to run off.
Ideally, if you expect rain, you will cancel the concrete placement. If you are not able to, then plan on covering the concrete with plastic sheeting as soon as possible after placing and floating the concrete. Uncover the concrete only if it begins to set too hard for adequate finishing, or if the rain passes by. Failing to cover the concrete may allow the cement paste to wash away, leaving very little workable material to finish, but trying to finish the concrete while water is standing on the slab will certainly allow the water to work into the concrete, and this will weaken it.
Keep in mind that the hydration process that causes concrete to set produces heat, so covering the slab while the concrete is wet may cause the set to speed up, and if the concrete remains covered until the concrete begins to set quickly, it will be difficult to achieve an acceptable finish.
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