A tack is a similar deed to the modern lease, and is a contract between a proprietor and a tenant (or 'tacksman') in which the tacksman could enjoy possession of the proprietor's land for a certain time on payment of a set rent. Indeed there are also leases to be found in the register of deeds. Only a tiny proportion of the many thousands of tacks that once existed were ever registered, however, and usually only if there was a dispute about the terms. In practice, if you are looking for a particular tack or lease, there is more likelihood of finding it among the estate papers of the landowner concerned.
A transcription from The Binns Papers illustrates a "Tack":
TACK of the small or vicar's teinds of Bynnis, etc., by Mr. William Powrie, minister of Abercorn, to the said Thomas Dalyell and his spouse, for the grantee's lifetime and for three years after his decease; paying therefor 3 1/2 marks yearly. Done at Bynnis, 21 December 1649. Witnesses, Henry Jamissone, servant to Lady Midhop*, James Mowat and Henry Stobie. (Signed) "Mr. William Powrie, parson and minister of Abercorn," and the witnesses.
* Margaret Hamilton, widow of Sir Alexander Drummond of Midhope