This could mean feeling too anxious to start a conversation with an attractive stranger they've just noticed at a party. Some shy guys, especially younger ones, have had a crush on someone for years and years, and have never worked up the nerve to talk to her.
The occasional time that someone they fancy talks to them first they often get too flustered, and the resulting brief conversation consists of some stuttered half-coherent answers on their part.
They may never ask them out, or put it off so long that by the time they do they're well into the Friend Zone.
He notices she seems like she's becoming attracted to him, and he blows things soon after.
Hanging out, going on a date, the idea is the same.
He sees this, gets nervous, and never conjures up the guts to speak to her. He can't work up the nerve to talk to her another time.
Another variation is when a guy runs into a girl, say because he sits beside her in a lecture, chats to her for a bit, and comes off well. A third scenario might be something more drawn-out, like a guy has a few conversations with a girl he never really thought of as more than someone friendly to talk to.
Sometimes a guy will be able to talk to someone he's interested in, often because the circumstances put them together, so the onus wasn't on him to approach anyone.
If they like someone they'll often feel too nervous to ask them out and face the awkwardness and risk of rejection that entails.
It's legitimately strong, not something where they can just take a deep breath and push through it.
Sometimes this nervousness shows up as the physical symptoms of anxiety.
Unsurprisingly, they're really nervous and hesitant about this as well. They may have a blatant, all-systems-go opportunity where the girl is clearly interested, but they can't manage to go through with it.