Explain the difference between: "If I work out, I will lose weight." "If I worked out, I would lose weight."

There is a very subtle difference and if you wanted to be picky I would suggest that the second sentence is technically incorrect!

Originally In the second sentence "worked" is past tense and you are combining it with "would" which is future tense. It would be better written as "If I had worked out, I would have lost weight", making the whole sentence "past" tense.

The first sentence is "all" future tense and is grammatically correct.

I WILL GO WITH FIRST SENTENCE ......... IT GIVES PROPER MEANING OF WORK OUT.......

Real situation andf unreal situation, Conditionals 

First sentence is very clear and it shows the condition of working out for losing weight in future. 

The second sentence is mixture of past and future. It has been well explained by Peter.

In the first sentence there is a good chance that the speaker will work out and lose weight , whereas in the second sentence it is unlikely that the speaker will be able to do either

Grammatically the first sentence is If (1), which is used to express possible future actions that may happen or not.

The second sentence is If (2), and it is, simply, used to express an imagined action, provided that a condition is fulfilled! And that is why the simple past "worked" is used in the if clause; whereas "would", a past future form of will, is used in the main clause i.e. the result clause.

Check this: If I were a fish, I would not get caught by a fisherman. This is just an imagination because I'm not a fish, and will never be! We use If (2) in such cases.

the part one of sentence described the meaning of if one or if conditions so the action may happen in present of time but the  part of sentence two it described an action happen in past so there are differ in meaning and grammertically

I agree with MR peter scott

if type one in English Language has a possibility so it may happen or may not in future , the third to if clause it have no chance to fulfill because it had already happened .