In a word, yes, if you are running an unincorporated business.
The T2125 is used to report business income. Previously, there was a separate form T2124, the Statement of Business Activities Form and T2032, the Statement of Professional Activities Form and you used the form that best represented how you made your business income.
The new form combines the two previous forms, so if you have either business or professional income, you'll fill out income tax Form T2125.
Like the previous forms it replaces, Form T2125 will be included in your T1 income tax form package.
Here's what Form T2125 looks like.
Note that Form T2125 is used to report any and all business income. Even if you haven't registered a business, you still have to fill out Form T2125 when you're completing your T1 Canadian income tax return if you have any business income at all.
See my Business Income Definition for more on what counts as business income in Canada.
Note, too, that if you have more than one business or professional activity, you need to complete a separate Form T2125 for each one.