One kilometre is exactly 1,000 metres. This is one of the few conversions on this site with nothing to round, nothing to approximate and no historical dispute — the prefix kilo means a thousand, and that is the entire relationship.
About the units
The metre is the SI base unit of length, defined since 1983 through the speed of light. The kilometre is not a separate unit at all but the metre with a prefix attached, and understanding that is the key to the whole metric system. The prefix kilo comes from the Greek khilioi, a thousand, and it was one of the original prefixes adopted when the metric system was created in revolutionary France in the 1790s. The same prefix does the same job everywhere it appears: a kilogram is a thousand grams, a kilowatt is a thousand watts, a kilojoule is a thousand joules. Learning one prefix teaches you every unit it attaches to, which was the explicit design goal.
The exact factor
The factor is exactly 1,000, so the conversion is a decimal shift: move the point three places right going from kilometres to metres, three places left going back. 5 km is 5,000 m; 0.75 km is 750 m; 12,345 m is 12.345 km. There is no rounding at any stage and no loss of precision, which is the practical advantage the metric system was built to deliver. The contrast with the imperial system is instructive: converting miles to feet requires the arbitrary 5,280, and converting feet to inches requires 12.
Where you meet this conversion
The conversion appears constantly in ordinary reading rather than in specialist work. Race distances are given in metres up to 10,000 and in kilometres above that, so a 5,000 m track race and a 5 km road race are the same distance described two ways. Maps switch between the two depending on scale. Altitude, depth and elevation are usually in metres even when horizontal distances are in kilometres, which is why a mountain profile mixes both axes.