This summary is not the rule. The rule text controls.
A lawyer generally can't be both the trial attorney and a witness in the same case. If a lawyer is likely to have to testify about important facts, that lawyer should not also be the one arguing the case in front of the judge or jury. This is because it can confuse the jury and creates a conflict between the lawyer's job of presenting evidence fairly and their role as an advocate for one side.
There are some exceptions. A lawyer can still act as both witness and advocate if the testimony is about something nobody disputes, if it only involves explaining legal fees charged in the case, or if removing the lawyer from the case would create a serious hardship for the client. These exceptions recognize that sometimes strict separation isn't practical or necessary.
This rule only applies to the individual lawyer who will testify. Other lawyers at the same law firm can still represent the client at trial, even if their colleague is called as a witness. However, this only works if there isn't some other conflict of interest problem, such as the firm representing conflicting interests or having confidential information from a former client that would affect the case.