Film speed refers to the sensitivity of film to light. Film sensitivity is measured by a set of standards established by the American Standards Association (ASA) and the International Standards Organization (ISO). For all practical purposes, these ASA and ISO numbers are interchangeable. In general, films with a higher sensitivity (larger number) have coarser grain and do not register detail as well as films with lower sensitivity (lower number). Slower film speeds equate to better detail and sharpness in your photographs, and need to be exposed for longer than faster film speeds. The typical number series for film speed on a rangefinder camea is:

25   50   100   200   400   800

Each stop on this scale is twice as sensitive to the number on the left, and half as sensitive as the number to the right.
Remember to set the camera for the same number that is on the film to get the correct exposure readings from your light meter.