what is that problem with the battery that it show a message consider replacing ur battery
Batteries have finite life. After a while with usage, especially if you deplete your battery a lot (lithium batteries do not like this, they don't like heat either), it will lose capacity. The operating system you are using has identified that your battery has lost enough capacity to suggest you to replace it.
On average, with moderate usage, you can expect a lithium-ion (or polymer) battery to lose half of its original capacity in 18-24 months.
OR, what happens also, but more rarely is that one of the few cells in the battery has died or is dying prematurely. This happens because modern lithium batteries for laptops are multi-celled, and each cell is only connected to other cells electrically and not chemically, so a manufacturing defect, for example, tends to show up in one of the (6 usually, or 9) cells in a laptop, and cuts the battery life span significantly.