The weather side is that side of a vessel upon which the wind blows.
The lee side is that side of the vessel which is farthest from the wind, and is, in a fore-and-aft rigged craft, the side on which the sail is stretched.
Now you will understand that, in consequence of the vessel turning round and presenting first one side to the wind and then the other, these names are continually shifting from side to side.
For instance, if the wind is blowing on the port—left-hand side—that is the weather side, and the starboard—right-hand side—is the lee side. Turning the vessel round, so that the wind blows on the starboard side, that becomes the weather side, and the port side becomes the lee side.
Having these sides and their names clearly fixed in your mind you will be able to understand what a tack is.
When a vessel is sailing on a wind we say she is on the port tack or starboard tack, meaning the way she is heading in regard to the direction of the wind. This tack is determined by the side upon which the wind blows.
A vessel is on the starboard tack when the wind strikes upon her starboard side and the boom of her mainsail is over on the port side. The reverse of this puts her on the port tack.
Or, to be more concise, when the starboard side is the weather side the boat is on the starboard tack. When the port side is the weather side the boat is on the port tack.