DM on Instagram stand for Direct Message. This basically a direct message to a person via Instagram. With DMs a person can share either images or videos taken by your camera or you can share images and videos from Instagram to a friend through DM.
To access DMs press the small arrow looking button on the top right corner of your home screen of your Instagram account. You will be notified that you received a DM when the arrow in the right corner has changed to a pink circle with the number of messages in the circle. If you receive a dm from a non follower, Instagram will give you the choice to accept or decline the conversation. You can also use DMs to create Instagram groups up to 15 people, where users can have discussions at the same time.
Slide into DMS
Sliding into DMs is a term more used since you can have an open conversation with anyone on Twitter by replying to a tweet or putting @username in front of your tweet, DMs are often used for conversations that you and the other user would rather keep just between the two of you. The expression “slide into your DMs” implies that it takes someone who’s very smooth and bold to privately message a stranger or acquaintance online.
In many cases, it’s an expression often said to represent the way an overly confident or even flirtatious user might contact someone they’re interested in. “Slide into your DMs like…” is sometimes accompanied by something visual – a photo or video to convey the right action or feeling.