In all-grain brewing, the mash is the first step in the brewing process.
The mash is conducted by mixing hot water with the malt (ie grains such as malted barley and/or wheat). A great ratio to conduct the mash is 2.5 L of water for every kg of malt (or about 1.25 qt/lb).
For example, if your recipe calls for 6 kg (13.2 lb) of malt, then you'll want to add 15 L (~4 US Gal) of strike water.
The term 'strike water' refers to the water that you will add to the mash tun along with the grains to begin the mash.
Your mash tun and the grains used in your mash will likely be room temperature when beginning the process. In order to hit your target mash temperature, the strike water needs to be higher to account for heat loss from the grains and the inner lining of the mash tun.
The exact temperature of your strike water will depend on your specific brewing set-up, but a good rule of thumb is to strike with 73°C - 77°C water (163°F - 170°F), so that your mash temperature settles in around 67°C (153°F), but anything between 64°C - 71°C (147°F - 160°F) is fine.
The most common type of mashing is single infusion. This simply means that you add your malt to the mash tun and pour your strike water on top of it. Once that you have done that, be sure to stir the mash thoroughly to ensure that there are no pockets of dry grain. To get the most efficient sugar extraction, you'll want to make sure that all of the grains are thoroughly saturated before you stop stirring.
If your mash temperature settles below 64°C (147°F), try adding some near-boiling water to raise the temperature. If your mash temperature is above 72°C (160°F), try adding some colder water or ice to bring the temperature down.
Some beer styles dictate multiple mash temperature rests to activate various enzymes, but a good place to start is to conduct a 60-minute mash. This will allow enough time for the naturally occurring enzymes within the malt to fully convert the starches in your mash to sugar. Be sure to check your mash a few times throughout the 60 minutes to ensure that the temperature stays constant and make any adjustments as necessary.