The iPhone refuses to run unsigned apps. In iOS, the primary way to distribute released apps to the public is the App Store. However, most companies don't want to distribute their internal apps for employees on the App Store, because that would mean that their internal apps are not-so-internal any more. Apps distributed via this method have to be cryptographically signed by Apple using Apple's certificate. Thus, Apple introduced Enterprise Certificates for companies to sign their own apps for installation on their employees' or their own devices.
If the person you received the app from is not your employer, you should almost certainly not install the app on your device with very few exceptions (no, "from a big company" is not one of them. If anything, it actually reduces the chance that the app is legitimate). The fact that it's being distributed using an enterprise certificate instead of through the App Store (or TestFlight, if it's a beta app) means that for some reason, the company writing the app does not want to submit it to Apple for review. Normally I'd say this could be due to cost issues, but the cost to get an Enterprise Certificate is over three times greater it costs to get an app onto the store. It's very likely then that there's some "feature" in the app that the company does not want either you or Apple to know about—which could very well be malware.
Also, if the cause was using Private APIs I'd suggest that the company just distribute it by sending you an IPA and having it sideloaded using AltStore or the like, though I'd still be wary if you haven't either read the source code or have a significant amount of trust in the sender. Allowing an Enterprise Certificate onto your device comes with several security risks because it usually assumes an employer-employee relationship and thus a certain amount of trust in the certificate holder.