John is, of course, absolutely right. But it can be explained in somewhat simpler terms, which might help a few of the people reading here.
When this kind of battery charger is first turned on, it immediately applies full power. We're talking amps here, not volts. The battery might be 12.2 volts, or 11.9 volts or whatever, the charger will charge at maximum, which means if it's a 20 amp charger it will provide 20 amps, if it's a 50 amp charger, it will provide 50 amps, etc. This is the bulk stage or "Constant Current" John mentioned.
As it does this, the batteries voltage will start to rise. The battery charger monitors the battery's voltage while it's providing maximum current. Eventually, the battery's voltage will reach the so-called "set point" that is either built in or that you chose from various available options. Let's say it's 14.7 volts. At this point, the charger begins to cut back the amount of amperage it provides in order to keep the batter's voltage from going any higher. If it didn't do this, the battery voltage would just keep going up, up up, until the battery was damaged from being "overcharged" This is the absorption stage or "Constant Voltage" stage. The battery remains at 14.7 volts as the number of amps the charger provides continues to fall.
Eventually, the battery charger decides that the battery is "fully charged". As Sternwake like to point out, most of them do this very badly, and it really isn't fully charged at this point, but the battery charger THINKS it is. It then cuts back to the so called float stage, where it provides a minimal amount of current at something like 13.4 or 13.6 volts.
An equalization charge is a deliberate overcharge. The charger provides full amperage until the battery voltage is driven up to the 16 to 16.5 volt range. This causes some of the battery's electrolyte to actually "boil off", which is why it is only done to batteries where you can replace the missing electrolyte with distilled water as needed, NEVER sealed batteries. This equalization does two things, it causes the electrolyte to mix thoroughly, and it causes any sulfation on the battery plates to hopefully convert back to lead.
(In flooded batteries, electrolyte can stratify, which means the sulfuric acid can be stronger on the bottom than on the top, which causes the lead plates on the bottom to be affected more and get thinner than the top of the plates. A properly equalized battery will last longer than a battery that is never equalized will.)