Paul is sending Timothy to the Philippians, and soon. His desires, however, and even his expectation, is that he himself will be able to go to them as well.
You can see his desire in Philippians 2:23-24:
Paul is sending Timothy to the church in Philippi. Timothy is concerned for the believers in Philippi. His efforts for the gospel are first-rate. He is the logical choice to send.
And, in fact, Paul expects to do just that, as soon he sees how things will go with him. Just what he means by this is uncertain.
The most obvious conclusion is that Paul’s legal problems are being referred to, and that may well be the case. He is in prison, and needs the help that an outsider, such as Timothy, could give. And with his trial looming in the uncertain future, it may be expedient to only send Timothy once things in that area have been nailed down. The added benefit to this is that Timothy would be able to bring the Philippians news of Paul’s continued imprisonment or imminent release.
Another consideration, though, is that Paul may have been referring to personal needs, or a concern for the church. It may surprise many that at this point, Paul is probably in his 70s or even 80s. And prison is no easy thing for anyone, even without the frailties of age. Granted, Paul’s imprisonment in Rome may have been more of a “house arrest,” but he still would have needed the assistance Timothy could give, and he may not have had anyone else that he could depend upon.