Estella says this to Pip when he protests against her throwing herself away by marrying a "brute" like Drummle. What she means is that she has been taught all her life by Miss Havisham to be cold and disdainful towards other people, especially men. Estella cannot feel love and affection the way others can: her heart, due to her training by Miss Havisham, is icy. She can't even love Miss Havisham, the woman who raised her. Drummle may be a brute to her, but Estella will repay it with scorn and lack of concern. She says to Pip:
Should I fling myself away upon the man who would the soonest feel (if people do feel such things) that I took nothing to him? There! It is done. I shall do well enough, and so will my husband. As to leading me into what you call this fatal step, Miss Havisham would have had me wait, and not marry yet; but I am tired of the life I have led, which has very few charms for me, and I am willing enough to change it.
In other words, she realizes it would be worse to give herself to a man, like Pip, who could "feel" and would be hurt by her lack of ability to love him in return. Estella is aware of what she lacks and is humane enough, in a rational way, to want to spare a deserving person pain.
When Pip breaks out in impassioned love talk about how much she is a part of him, Estella simply looks at him with "a ghastly stare of pity and remorse."