The IMSI is the number a mobile network uses to know who you are. It identifies the subscriber, meaning the line and the account behind it, rather than the physical handset. Every SIM card and every eSIM profile carries one, and the network reads it during authentication to decide whether you are allowed on and what your plan lets you do. You almost never see it, and that is by design: it is a private identifier that stays inside the SIM and the operator’s systems.
The key distinction to hold onto is IMSI versus IMEI. The IMSI identifies you, the subscriber; the IMEI identifies the device, the phone itself. Swap your eSIM profile into a different phone and the IMSI travels with it, because it belongs to the line. Put a different SIM into your phone and the IMEI stays put, because it belongs to the hardware. Networks look at both: one to bill and authenticate the right account, the other to check the handset is not blocked.
The IMSI does its most interesting work when you travel. When your phone latches onto a foreign network, that visited network reads the IMSI, sees which home operator it belongs to, and sets up the roaming relationship so your line works abroad. With a local travel eSIM the profile carries an IMSI tied to a network in the region you are visiting, so you connect as a local subscriber rather than a guest. That is part of why network selection behaves differently on a travel profile than on your home SIM.
For practical purposes you rarely touch the IMSI directly. It provisions itself when the profile installs, and if you follow our install and activate guide you never have to type it. Just know that when a network authenticates you in seconds on landing, the IMSI is the number doing the introductions.