A shared IP is a sending IP address used by many senders at once. Google Workspace, Microsoft 365 and most email services send from large pools of shared IPs.
The upside
The provider watches the pool, removes bad senders and keeps the IPs' reputation high. A new sender benefits from that straight away, with no IP to warm up.
The downside
On a poorly run service, other senders' spam can affect you. Large providers keep that risk low; small, cheap sending services are where it shows up.
For cold email
Shared IPs at a major mailbox provider are the right choice for cold email: your domain reputation does the work, and the provider handles the IPs. It's why mailzy's mailboxes are Google Workspace mailboxes.