It is important for all guitarists to be able to tune their guitar. Even with all the digital tuners and apps available you will also need to learn how to tune by ear. Whether the battery in your tuner dies or it is simply not within reach from where you are practicing, tuning by ear is an essential part of playing the guitar.
If your guitar sounds like it’s out of tune when you strum a chord, it probably is. It can be difficult to know which string or strings are out of tune. On a digital tuner each string on the guitar is tuned to 440 Hz. If you tune it by ear you may be a little higher or lower than 440 Hz but that’s OK if your playing the guitar by yourself. The important thing is that the relative distance between each string is correct. When playing in a band or along with recorded music you should be exactly at 440 Hz on a tuner.
Below is a chart I made that explains how to tune by ear.
I hope this helps.