Truth is it can mean a lot of things to a lot of different people. Like many things with BDSM, every relationship is different. People will tell you that there are different classes or types of collar while others will swear that they are only for slaves.
The only thing that I can tell you is what it means to me. In some cases such as events, it can mean the protection of having someone watching out for you even if they are not in the room. In others such as a day collar it can mean a subtle constant reminder of my love and Dom.
I feel I should explain something here. My engagement ring is a symbol of our relationship aside from the D/s. A day collar symbolizes the D/s side of the relationship. It is important to note that he and I have a promise that even if the D/s side ends for whatever reason, our relationship is safe.
Hopefully my Dom will weigh in on the subject himself.
And so I shall. As my wonderful sub has stated, the collar can mean many different things to many different people. Nearly everyone involved in the BDSM lifestyle has a different take on the subject. That being said, it is my belief that a collar should not be offered lightly. I do not think of it as a fashion accessory or a symbol of station. It is, first and foremost, a promise. It symbolizes the love and responsibility that the Dom wields in regards to their sub.
By collaring a sub, the Dom has accepted that they are solely responsible for the physical, mental, and emotional well-being of their sub. This is regardless of the specific roles in their BDSM relationship. Even if they only utilize a collar for the purpose of doing a scene, or while they are in the bedroom/at home/etc., as long as the collar is being worn, the Dom must ensure the needs of their sub are met.
It is also important to mention that simply removing the collar does not absolve the Dom from these responsibilities. Especially after a scene or punishment when aftercare is definitely required. A collar should never be used as a means to get what you want without repercussions.