Hi I am a guitar beginner, can someone help me to understand what does that number (3 under notes) mean?
Since you're a beginner, I would assume you're working in 4/4.
If that's the case, the "3" that connects the eighth notes makes them an eighth note triplet. This means that you fit the E, D, & C all into the timing of one quarter note (in 4/4).
This video explains it fairly well for a beginner.
When I first started learning how to count, it helped if you say "triplet" (pronounce "trip-ah-let" for the timing) in the time of one beat. (So you would tap your foot or finger or have a metronome, and make the syllables individual notes. i.e. in your example, "Trip" = E, "ah" = D, & "let" = C.) That being said, once you're comfortable, you should break that habit and just get used to the timing itself.
If you're new to reading music and still don't fully grasp note values/length in a given time signature, you should really start with the basics (whole, half, quarter, eighth, etc.) then you can tackle tuplets.
Hopefully that makes some semblance of sense.