Following Henry's post, timing means that you need to click on the timing setup panel in the timing section of a map and put down BPM and offset of the song. As defined by the osu wiki, offset the amount of time in milliseconds between the start of the audio file and the song's first downbeat. Songs that come from other music games usually have their BPM provided, and all the player has to do it fine the offset. Otherwise, you need to find the BPM and offset yourself. There are people in the modding forums, however, willing to help you find it.
When you find the correct BPM and offset for your map, you then go to the mapping field (by clicking on it, or by pressing f1 for quicker access) and select how you would like to divide one beat within a measure. This is found on the top right corner of the mapping field.
For example, if I map something that's 120 BPM in 4/4 meter and set the beat divider by 1/4, this means each beat (one beat meaning each white tick on the timeline on top) will divided into fourths with a line appearing in the timeline every 0.125 seconds.
More can be read in detail here in the osu wiki:
wiki/en/Beatmap_Editor/Timing