The Hebrew term Kavvanah is a complex one that means, among other things, intention, attention, purpose, devotion, and concentration of thought during prayer or in the performance of religious commandments.
There is a well-known dictum stressing the importance of Kavvanah: "Prayer without Kavvanah is like a body without a soul.”
This means that prayer, which is the speech addressed by man to his Creator, loses its essential quality when it becomes solely the recitation of words, without inward attention to the meaning of the words being spoken.
Obviously, spontaneous, personal prayer, uttered in time of need, is expressed with Kavvanah, because it springs entirely from the heart of the worshipper at that particular moment; such words of prayer (if they are in fact articulated verbally, and the one praying does not suffice with thought alone) are an expression of earnest intention.
The problem arises when there is a fixed order of prayer, recited from the Siddur or learned by rote, so that one may reach a stage where the words are recited almost mechanically, without thinking about their real meaning or significance.
The problem is not a new one; the sages of the Mishnah already referred to it by stating that "one who makes his prayer a fixed routine, his prayer is not considered a supplication."
Some of the greatest teachers of mishnaic and talmudic times complained of difficulty in maintaining a level of inner concentration during prayer.
In later generations, it was even conceded that, in general, "at no time are we ever sufficiently concentrated” as a result of which certain practical guidelines regarding prayer were proposed.
Nevertheless, Kavvanah in prayer is not only a spiritual desideratum for the worshipper: it is also a binding halakhic requirement.
The following is the formulation of the Shulhan Arukh, which generally deals with the formal halakhic details of Jewish religious life:
The worshipper must inwardly intend the meaning of the words uttered by his
lips, and imagine himself to be in the presence of the Shekhinah, and should
remove any disturbing thoughts, until his mind and heart are pure for prayer. He
should think that, were he standing before a king of flesh and blood, he would prepare
his words carefully and address them well in order not to fail in his attempt.
All the more so when he is standing before the King of Kings, blessed be He, who
searches our innermost thoughts. For this reason, pious and worthy men of old
used to seclude themselves and concentrate their minds in prayer until they were
able to transcend their physical being and strengthen the power of their intellect so
as to attain a level close to prophecy.
While not everyone is required, or can even be expected, to attain such a high level, it certainly sets a certain standard in regard to Kavvanah, and even those unable to reach such heights should not remain satisfied with being merely "like a twittering bird"-in the derogatory words used by certain sages–or like a parrot repeating sounds that have no meaning for it.
–Rabbi Adin Steinsaltz
From A Guide to Jewish Prayer by Rabbi Adin Steinsaltz