This is a Letter of Marque.  It was official government permission to attack enemy ships.  This particular one was granted to Captain Robert Kidd, a rather famous Privateer.

William Rex,
William the Third, by the grace of God, King of England, Scotland, France and Ireland, defender of the faith, &c. To our trusty and well beloved Capt. Robert Kidd, commander of the same for the time being, Greeting: whereas we are informed, that Capt. Thomas Too, John Ireland, Capt. Thomas Wake, and Capt. William Maze or Mace, and other subjects, natives or inhabitants of New-York, and elsewhere, in our plantations in America, have associated themselves with divers others, wicked and ill-disposed persons, and do, against the law of nations, commit many and great piracies, robberies and depredations on the seas upon the parts of America, and in other parts, to the great hindrance and discouragement of trade and navigation, and to the great danger and hurt of our loving subjects, our allies, and all others, navigating the seas upon their lawful occasions. Now know ye, that we being desirous to prevent the aforesaid mischiefs, and as much as in us lies, to bring the said pirates, free-booters and sea-rovers to justice, have thought fit, and do hereby give and grant to the said Robert Kidd, (to whom our commissioners for exercising the office of Lord High Admiral of England, have granted a commission as a private man-of-war, bearing date the 11th day of December, 1695,) and unto the commander of the said ship for the time being, and unto the officers, mariners, and others which shall be under your command, full power and authority to apprehend, seize, and take into your custody as well the said Capt. Thomas Too, John Ireland, Capt. Thomas Wake and Capt. Wm. Maze or Mace, as all such pirates, free-booters, and sea-rovers, being either our subjects, or of other nations associated with them, which you shall meet with upon the seas or coasts, with all their ships and vessels, and all such merchandizes, money, goods, and wares as shall be found on board, or with them, in case they shall willingly yield themselves; but if they will not yield without fighting, then you are by force to compel them to yield. And we also require you to bring, or cause to be brought, such pirates, free-booters, or sea-rovers, as you shall seize, to a legal trial, to the end they may be proceeded against according to the law in such cases. And we do hereby command all our officers, ministers, and other our loving subjetcs whatsoever, to be aiding and assisting to you in the premises. And we do hereby enjoin you to keep an exact journal of your proceedings in execution of the premises, and set down the names of such pirates, and of their officers and company, and the names of such ships and vessels as you shall by virtue of these presents take and seize, and the quantities of arms, ammunition, provision, and lading of such ships, and the true value of the same, as near as you can judge. And we do hereby strictly charge and command you, as you will answer the contrary at your peril, that you do not, in any manner, offend or molest our friends or allies, their ships or subjects, by colour or pretence of these presents, or the authority thereby granted. In witness whereof, we have caused our great seal of England to be affixed to these presents. Given at our court in Kensington, the 26th day of January, 1695, in the 7th year of our reign.

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