You may have heard of the terminology "winging scapula" whether in the context of fitness/movement, posture, or even in regards to instrument setup and performance. What exactly is it?
Let's start with some background: your scapula (shoulder blade) is one of three bony structures of the shoulder girdle, those being scapula, humerus, and clavicle. These bones and subsequent muscles work together to both stabilize and mobilize, depending on the joint action or movement (AKA. they all have the potential to move) However, not all body actions require movement of all three joints- some movements just ask the arm bones to move, others ask for elevation of the scapula and clavicles, and other movements integrate all three bones into collaborative movement. A "normal" scapula (is anyone really "normal" though?) would lie against the ribcage in rest, and in movement, would have a full range of motion, permitting full overhead range (shoulder flexion, as seen in this video above!). A winging scapula may protrude at rest, and when the body moves in different ways, the scapula protrude on the back and move significantly. One of the classic tests for winging is a wall push-up and quadraped push-up, which determines whether the motion is coming from the arm bones or if the whole arm/scapula/collarbone unit is moving unnecessarily off the back. The primary muscle that opposes this whole winging thing is the serratus anterior, which protracts the scapula (brings scapula wide) amongst other things. This can also tie in with trapezius and pec imbalances (which, let's be honest, most of us have either from instrument playing, desk sitting, or hyper-kyphosis).
So what's the big deal with winging scapula? Many people have lost the ability to individualize arm movement (humeral movement) from scapular movement, and don't have the ability to protract (or stabilize or propriocept) the scapula. For some people, that translates to pain, lack of range, and instability, and for others, just "tightness" and difficulty in different exercises. It's something I've worked on a LOT these last two years, and it's changed the way that I teach and practice (meaning that I rarely teach chaturanga-vinyasas because it's a winging scapula nightmare).
To get started on your scapula assessment, took a look at your backside in the mirror- notice how your scapula orient on the back in standing. Then try different arm movements and notice the movement in your scapula (or lack of movement). Try the classic wall pushup and have someone observe (or phone video) your scapula. (This video shows winging scapula in the wall pushup! Notice howin both versions, she has excessive scapular retraction, meaning the scapula draw together on the back as the arms bend.) My favorite exercises to strengthen serratus anterior are planks with protraction, protraction/retraction exercises in quadraped, and developing an awareness of the back body in all movements, especially in quadraped and plank positions, which are essential to many different movement disciplines.