That's up to you! You can select which videos you want to be accessed for free and which videos you want to have locked and secured for paying subscribers only. Videos that are free can be watched by anyone, without limit. Videos that are locked cannot be watched by anyone other than a paying subscriber to your channel.
There are benefits to having a select number or certain type of content for free. By configuring your video catalog to have a mix of free content and paid content, new viewers can discover your brand, watch your videos, and determine if a subscription to your channel is right for them. This, in tandem with locking certain content and providing exclusive access to that gated content, helps drive users to subscribe to your channel to get access to that content.
For example, uploading a trailer or teaser as a free video is a great way to show potential subscribers what content they could watch, catch their interest, and entice them to subscribe to your channel to get access to that content.
Some creators upload the first or first few episodes of a series for free, and lock the remaining episodes. Other creators upload free "behind-the-scenes" or "blooper reels" to gated content. Free content is also often used to share across social media and drive that audience to your channel's page and subscribe.
It's up to you to decide what content you want for free, or if you want free content at all! You're in control of the way visitors and subscribers have access to your content. And don't worry-- regardless of whether you've selected a video to be free or locked, it will still be discoverable through search engines such as google search, to help drive a greater audience to your channel.