What does it mean when a reader says "I feel someone's energy"
I would say a high percentage of readers have what can be described as empathy. Empathy for me is the ability to allow myself to "feel" what others feel on demand. In a sense, if someone hurts say like as in a toothache, then my teeth start hurting. If someone has heart problems, then I have feel pain in my heart region, if someone is having lung trouble I will start having difficulty breathing, and so on. The same kind of empathy goes for anxiety, confusion, depression, love, and anger. When I get these feelings though, at times I just rather not feel them, (they can get intense with me to a point that I end up too emotional for my family) so I have asked Spirit to give me thoughts, pictures, or words for emotions instead of feeling them.
So when a reader says that they feel someones energy, then that reader most likely has developed empathy to a point that he or she can "feel" others energy on demand as well.
Your other question on " when a reader says that I will feel someone's energy what does that mean and how?"
That is really close to the answer above. Reader's like myself develop this empathy. I believe it is actually one of the first gifts that we get. Also there is telepathy. Although I don't see it used like in the movies, everyone seems to have a bit of it. So when a reader says, that "you" will feel it, they are speaking of either empathy or telepathy, and or a mixture of both coming into play for you.
The way I have showed people that they are already using telepathy is that I ask them "Did you ever have someone say that I was just thinking about you when you called."This started by you having an urge to pick up the phone and call. Just before you called that someone, you would cast out a mental fishing line and "think" that they will answer. They in turn "felt" the mental pull. They then started to think about you. So this relates to the original question on how you can "feel" someone's energy.
Another example: When someone is thinking about you, "You" will out of the blue, while doing something totally different with no thought what so ever of that person, start thinking of that person. such as reading a book, you just get to an interesting part, action going on, then a thought comes into your mind about kissing this person. Your going well that was random, where did that come from. That is telepathy going on. If you get warm desires along with the thought, then that is empathy and telepathy happening. Once you know it is happening then you know how and what to look for. By the way these thoughts and feelings are never wrong. (even if someone says they don't care, if it comes to you randomly, THEY CARE!)
This can be fun or frustrating. It happens alot with teenagers, then we sorta grow out of it. If you want to practice, get a friend to join you in with this, and say ok sometime today, I'm going to think of you randomly. If I catch you unaware and get in your thoughts write down the time and thought that came through, and do the same for your friend. Have fun and let me know some of your experiences. Hope this helps