Impulse is force applied times amount of time it is applied for.

In other words, impule is a “compound” quantity, it has two parts 1) the force applied and 2) how long it is applied for.

When you apply a force on some object, you give it an acceleration. The longer you apply the force, the longer the object keeps on accelerating for, the more its velocity changes. Thus applying a force for a certain amount of time, results in a change in velocity for the object (the mass).

Looking from another perspective, the amount of velocity change that an impulse causes on an object depends on the mass of the object.

Thus, when you apply an impulse (a force for some time) on an object (mass), the object’s velocity changes by some amount; the change in velocity is proportional (linear) to the quantity $\frac{I}{m}$

In other words, the change in velocity is directly proportional to $I$ and inversely proportional to $m$

Let’s make it concrete. Let’s say we have a 1 kg object at rest. We apply an impulse of 100 to it. How much would the velocity change by?

Since the object started at speed 0, it will be up to 100 m/s now.

What if the object was 2 kg instead of 1? Well, then the speed would only change by 50 m/s. The heavier the object, the less this impulse will change its velocity.